There exist many Laser Doppler Velocimetry devices (LDV) (also known as laser Doppler anemometry, or LDA) to measure the speed and length of moving parts. These devices employ a technique for measuring the direction and speed of material that is processed.
The LDV crosses two beams of collimated, monochromatic, and coherent laser light in the flow of the material being measured. The two beams are usually obtained by splitting a single beam, thus ensuring coherency between the two and have the same polarity and exit the device at an angle. The two beams cross at some standoff distance from the device.
Where the beams cross (intersection) an interference pattern is created. At the beams intersection (the focal point of a laser beam), they interfere and generate a set of straight fringes.
A sensor is then aligned relative to the intersection such that the fringes are perpendicular to the directional movement of material. As material pass through the fringes, they reflect light (only from the regions of constructive interference) into a photodetector (typically an avalanche photodiode), and since the fringe spacing d is known (from calibration), the velocity can be calculated to be u=f×d where f is the frequency of the signal received at the detector.
Since the beam angle is fixed and the wavelength is constant, the distance between the fringes is known and is constant. As particles on the measurement surface move through this interference pattern, a time varying signal is created and measured by the device and converted to speed and distance. It is the light that scatters off of the light stripes of the fringe pattern that generates the signal. This signal is received by the APD (Avalanche Photo Diode).
The particles must be big enough to scatter sufficient light for signal detection (good signal to noise ratio) but small enough to follow the flow. By analyzing the Doppler-equivalent frequency of the laser light scattered (intensity modulations within the crossed-beam probe volume) by the particles within the movement, the local velocity of the material can be determined. The area of interest within the material field is sampled by a crossed-beam in a point by point manner.
While the above system works well on many surfaces, problems can arise when the surface is smooth and shiny. As the surface gets shinier the ratio of reflected light to scattered light increases. The speed information is only in the scattered light. As the reflected light increases, the APD gain decreases. It can decrease to the point where the scattered light can no longer be detected. In extreme cases, the APD can actually saturate due to too much light. In both of these cases, there is no measurement.
Another effect of too much reflected light is that light can feed back into the laser diode and cause it to mode hop. A mode hop is a wavelength change which affects the measurement accuracy. The diode can even get into a state where it is constantly mode hopping and this can result in no measurement.